Much of ceramics processing utilizes fine powders in combination with organic additives to form a ceramic precursor known as a “green body,” which is subsequently fired or sintered to give the final ceramic product. Typically, the green body contains a polymer binder which helps to maintain the structural integrity of the green body. When present in relatively high amounts, the polymer binder fills the void space between powder particles. The polymer binder is then removed or “burned out” and the structure is sintered to a full density component. Ceramic structures having simple geometries such as circular cross sections and complex geometries such as honeycomb structures have been manufactured.
One method for the fabrication of ceramic bodies is to formulate the ceramic-forming powders into a thick paste at very high solids loading, typically above 55% by volume, and to extrude the paste to form the green body. Most such pastes must possess at least 55% solid loading by volume to allow the extruded green body to maintain its shape and to be sintered to full density. However, extrusion of compositions having such high solid loading can present a number of technological challenges. First, the thick paste can be difficult to compound without the introduction of voids. Second, the thick paste can cause die clogging, i.e., it may not readily flow through a nozzle or extrusion die, making the process difficult to carry out and to control. Also, pressure increases caused by highly viscous pastes within the nozzle or extrusion die can generate wear on the apparatus.
The use of compositions having lower solid loading by volume results in slurries of lower viscosity and homogeneous consistency which could flow through a nozzle or extrusion die with very low probability of clogging. Also, such compositions are less likely to cause back pressure within the apparatus. However, upon exiting the nozzle, such slurries would likely immediately lose their shape, rather than retaining at least in part the shape of the die through which they were extruded, as their viscosity is typically not much greater than that of water. Further, the low starting density of such slurries may make it difficult to sinter the ceramic body to full density.
Accordingly, improved methods for the formation of ceramic structures are needed.